A typical Reactive Ion Etch (RIE) plasma processing chamber includes a radiofrequency (RF) bias generator, which supplies an RF voltage to a “power electrode”, a metal baseplate embedded into the “electrostatic chuck” (ESC), more commonly referred to as the “cathode”. FIG. 1(a) depicts a plot of a typical RF voltage to be supplied to a power electrode in a typical processing chamber. The power electrode is capacitively coupled to the plasma of a processing system through a layer of ceramic, which is a part of the ESC assembly. Non-linear, diode-like nature of the plasma sheath results in rectification of the applied RF field, such that a direct-current (DC) voltage drop, or “self-bias”, appears between the cathode and the plasma. This voltage drop determines the average energy of the plasma ions accelerated towards the cathode, and thus the etch anisotropy.
More specifically, ion directionality, the feature profile, and selectivity to the mask and the stop-layer are controlled by the Ion Energy Distribution Function (IEDF). In plasmas with RF bias, the IEDF typically has two peaks, at low and high energy, and some ion population in between. The presence of the ion population in between the two peaks of the IEDF is reflective of the fact that the voltage drop between the cathode and the plasma oscillates at the bias frequency. When a lower frequency, for example 2 MHz, RF bias generator is used to get higher self-bias voltages, the difference in energy between these two peaks can be significant and the etch due to the ions at low energy peak is more isotropic, potentially leading to bowing of the feature walls. Compared to the high-energy ions, the low-energy ions are less effective at reaching the corners at the bottom of the feature (due to charging effect, for example), but cause less sputtering of the mask material. This is important in high aspect ratio etch applications, such as hard-mask opening.
As feature sizes continue to diminish and the aspect ratio increases, while feature profile control requirements get more stringent, it becomes more desirable to have a well-controlled IEDF at the substrate surface during processing. A single-peak IEDF can be used to construct any IEDF, including a two-peak IEDF with independently controlled peak heights and energies, which is very beneficial for high-precision plasma processing. Creating a single-peak IEDF requires having a nearly-constant voltage at the substrate surface with respect to plasma, i.e. the sheath voltage, which determines the ion energy. Assuming time-constant plasma potential (which is typically close to zero or a ground potential in processing plasmas), this requires maintaining a nearly constant voltage at the substrate with respect to ground, i.e. substrate voltage. This cannot be accomplished by simply applying a DC voltage to the power electrode, because of the ion current constantly charging the substrate surface. As a result, all of the applied DC voltage would drop across the substrate and the ceramic portion of the ESC (i.e., chuck capacitance) instead of the plasma sheath (i.e., sheath capacitance). To overcome this, a special shaped-pulse bias scheme has been developed that results in the applied voltage being divided between the chuck and the sheath capacitances (we neglect the voltage drop across the substrate, as its capacitance is usually much larger than the sheath capacitance). This scheme provides compensation for the ion current, allowing for the sheath voltage and the substrate voltage to remain constant for up to 90% of each bias voltage cycle. More accurately, this biasing scheme allows maintaining a specific substrate voltage waveform, which can be described as a periodic series of short positive pulses on top of the negative dc-offset (FIG. 1(b)). During each pulse, the substrate potential reaches the plasma potential and the sheath briefly collapses, but for ˜90% of each cycle the sheath voltage remains constant and equal to the negative voltage jump at the end of each pulse, which thus determines the mean ion energy. FIG. 1(a) depicts a plot of a special shaped-pulse bias voltage waveform developed to create this specific substrate voltage waveform, and thus enable keeping the sheath voltage nearly constant. As depicted in FIG. 2, the shaped-pulse bias waveform includes: (1) a positive jump to remove the extra charge accumulated on the chuck capacitance during the compensation phase; (2) a negative jump (VOUT) to set the value of the sheath voltage (VSH)—namely, VOUT gets divided between the chuck and sheath capacitors connected in series, and thus determines (but is generally larger than) the negative jump in the substrate voltage waveform; and (3) a negative voltage ramp to compensate for ion current and keep the sheath voltage constant during this long “ion current compensation phase”. We emphasize that there can be other shaped-pulse bias waveforms that also allow maintaining a specific substrate voltage waveform shown in FIG. 1(b) (characterized by the nearly constant sheath voltage), and are hence capable of producing a mono-energetic IEDF. For example, if the electrostatic chuck capacitance is much larger than the sheath capacitance, the negative voltage ramp phase described in (3) above can be substituted with a constant voltage phase. Some of the systems and methods proposed below can also be implemented with these other shaped-pulse bias waveforms, and we will be making a special note of that wherever applicable.
While a single-peak IEDF is widely considered to be a highly desirable shape of IEDF resulting in improved selectivity and feature profile, in some etch applications an IEDF having a different shape, such as a wider shaped IEDF, is required.